The present invention relates to the sterilization of cryogenic liquids. The invention comprises a method and apparatus for providing a sterile cryogenic liquid in an efficient and economical manner.
Many new uses for cryogenic liquids have been developed which require that the liquids be sterile. For example, cryogenic liquids are used in the filling of aluminum cans with food or beverages. Because of its relatively low cost, aluminum would be desirable as a material for cans for food products. However, aluminum's softness has made it impractical to use in most cases. Unless the aluminum cans have been filled with a carbonated beverage, which creates pressure within the can, the aluminum cans cannot be stacked. The weight of a stack of cans literally crushes the cans near the bottom of the stack. A solution to the problems is to inject a small quantity of a cryogenic liquid into the can, immediately before the can is sealed. The liquid vaporizes almost immediately. The resulting gas, if unconfined, would expand to about 700 times the volume of the liquid. The gas trapped within the can will thus exert sufficient pressure to enable the can to withstand the weight of the entire stack.
Because the cryogenic liquid injected into the can directly contacts the food or beverage, it is absolutely essential that the liquid be sterile. The present invention fulfills this need for sterilization.
Another use for sterilization of cryogenic liquids arises in the medical field. Oxygen for hospital use is often stored in liquid form, in pressurized cylinders. The liquid is vaporized before being administered to a patient in gaseous form. It is obviously important that the oxygen which is supplied to an already weakened patient be free of unwanted particles or biological contaminants. The present invention fulfills this need as well.
The semiconductor fabrication industry is another area in which the present invention can be used. Semiconductor chips are manufactured a low temperatures, and must be grown under conditions of extreme cleanliness. It is therefore necessary that the cryogenic liquids used to maintain the temperatures be completely free of microscopic particles.
The present invention accomplishes the sterilization either by the use of ultraviolet radiation or by ultrafiltration. The basic concept of using ultraviolet radiation to sterilize fluids has been described in U.S. Pat. Nos. 2,044,279, 3,433,946, 4,156,652, and 3,894,236. However, of the cited patents, only the '279 patent involves the use of cryogenic liquids. The present invention provides a method which is more economical than those of the cited references. In the present invention, it is a cryogenic gas, rather than a cryogenic liquid, which is sterilized.
The use of microporous filters as a means of sterilization is also described in several patents. For example, U.S. Pat. No. 2,924,078 shows a method of purifying liquid oxygen through the use of filters and adsorbers. U.S. Pat. No. 3,739,593 discloses a gas separation system which includes means for filtering a liquefied gas. U.S. Pat. No. 3,653,220 shows a process for purifying helium, which process includes the use of molecular sieves to remove impurities by adsorption. U.S. Pat. No. 3,192,730 discloses a method of purifying liquid helium by passing the superfluid helium through filters. All of the disclosures of the patents cited in this and the preceding paragraph are incorporated by reference herein.
The present invention provides a method and apparatus for sterilizing a liquid cryogen in a two-phase procedure. The invention causes the sterilization to occur while the cryogen is a gas, thereby eliminating the need to insure against unwanted vaporization during the sterilization procedure. If sterilization is being done by filtration, then the fact that the sterilization is done while the cryogen is a gas makes it feasible to use less expensive filters to accomplish the purification.
The present invention can be used to sterilize any cryogenic liquid. The only change necessary, for a change in the liquid to be sterilized, is an adjustment of the temperatures and pressures used.